1. Field of the Invention
This invention relates to projection lithography processes and in particular electron projection lithographic processes.
2. Art Background
In device processing, an energy sensitive material, denominated a resist, is coated on a substrate such as a semiconductor wafer (e.g., a silicon wafer), a ferroelectric wafer, an insulating wafer, (e.g. a sapphire wafer), a chromium layer supported by a substrate, or a substrate comprising some combination of such materials. The resist is exposed by subjecting it to radiation in the desired image. This image is then developed to produce a patterned resist generally by immersing the resist in a suitable solvent or subjecting it to a plasma to remove selectively either the exposed or unexposed regions. The developed pattern is employed as a mask to process, e.g., etch, the underlying layer. The resist is then removed (for many devices), subsequent layers are formed, and the resist process is repeated to form overlying patterns in the device. In such repetition of the resist process, the pattern in the resist being processed is typically aligned (registered) relative to underlying patterns by using fiducial marks.
Various approaches have been proposed for the exposure of a resist with charged particle beams, e.g., electron beams, in the manufacture of submicron devices. (Submicron devices in the context of this invention is a body having a pattern with either lines or spaces smaller than 1 .mu.m). Electron beam exposure has been extensively used for the making of lithographic masks where the resist overlies a chromium layer that in turn overlies a quartz substrate. The image is produced by raster scanning an electron beam over the resist material in a single cycle and shuttering the beam at appropriate positions to produce the desired exposure image. This single cycle, raster process is capable of producing extremely fine features, but is generally too slow for making devices other than masks.
Alternative approaches have been proposed for exposing devices other than masks in suitable times. (Generally lithographic processing at least 30-60 wafers per hour is considered desirable where a wafer is a substrate typically from two to ten inches in diameter that is ultimately subdivided after fabrication into a plurality of devices). These exposure approaches are generally divided into proximity and projection procedures. In the former, a mask defining the image by absorptive/reflective regions and transmissive regions for the exposing energy is placed in close proximity to the resist. An electron beam is scanned over the mask or light is flooded onto the mask to expose the underlying resist in regions corresponding to transmissive areas of the mask.
In a projection approach, a lens is interposed between the mask and the resist. The mask is either the absorptive/transmissive type previously described or, alternatively, of a type that scatters in one set of regions to a greater extent than in a second to produce the desired image. The fluence traversing the mask is focused by the lens onto the resist to produce an image corresponding to the mask pattern.
In one specific approach to projection lithography, schematically illustrated in FIG. 3, (described in U.S. Pat. No. 5,079,112 dated Jan. 7, 1992, which is hereby incorporated by reference) a mask 20 is employed which scatters and/or reflects incident electrons 10 in a first set of regions 30 and scatters to a lesser extent, e.g., transmits, in a second set of regions 40. The electrons 10a traversing mask 20 are caused to converge at one or more convergence points by an electron optic projection lens 50. An area more transmissive, e.g., aperture 70, than the surrounding area 80 is positioned at such convergence point. Scattered electrons, 10b and 10c, do not converge at this point and are blocked while unscattered electrons 10a do converge at aperture 70 and emerge to expose resist on substrate 90.
Typically, in an electron exposure proximity printing procedure the electron beam is scanned electronically, i.e., by use of magnetic and electric fields over the mask. In one study published in Proc. 8th Syrup. on Electron and Ion Beam Science and Technology, 406-419 (1978), it is suggested that a very rapid line scan, i.e., faster than 0.2 ms, with repeated exposure of each portion of the resist to effect the desired dosage is useful to avoid localized heating, and thus localized expansion of the mask. In contrast, expansion of the entire substrate due to uniform heating is electronically compensated for during exposure. (See W. M. Moreau, Semiconductor Lithography, Plenum Press, New York, page 435 (1988)). Localized deformations produce errors (called overlay errors) in the placement, i.e., registration, of a resist pattern relative to an underlying pattern. Instead of scanning at a sufficiently slow rate to expose fully each region during one scan cycle, the rate of scanning is substantially increased and exposure is accomplished through a plurality of rapid scan cycles. Despite such precautions, higher acceleration potentials, although yielding enhanced resolution, nevertheless lead to rapidly increasing overlay errors.
For projection light lithography other schemes such as a step and scan procedure have been proposed. In this procedure, a portion of the mask is illuminated over a strip. The image of the entire mask is then projected on the resist by moving the mask and the wafer in opposite directions at a relative rate of speed that depends on the demagnification of the system. For example, if the system has a 4:1 demagnification (meaning a unit length of the mask is projected onto a corresponding one quarter unit length on the substrate) the mask is moved at a rate four times faster than the substrate.
The distance between mask and substrate in an electron projection lithographic system has, with progressing development, become significantly greater. This trend toward longer columns, i.e., longer distances between mask and substrate, has been driven by various optical considerations. Typically, a larger image field is available with lenses having longer focal lengths. Additionally, such longer focal lengths tend to reduce error associated with curvature of field. However, a longer focal length, depending on the extent of demagnification used, requires a longer column. (Demagnification is the degree of reduction of mask dimension of a feature to corresponding substrate size of the same feature). As shown in FIGS. 1 and 2, as the demagnification becomes greater for a conventional projection lens, the distance between the substrate and the mask increases. Thus, in such systems as shown in FIG. 1, for 1:1 demagnification the distance, 5, between the mask, 1, and projection lens, 3, (represented by a diagrammatic single lens) is approximately the same size as the distance, 7, between the projection lens and the substrate, 9. Similarly, in a corresponding 4:1 projection system, the proportion of these two distances is correspondingly 4 to 1. As a result, the desire for longer focal lengths and increasing demagnification yields increasing column size.